Abstract on the Aspen Association of Northern Michigan · 2018. 10. 2. · association have been created and maintained by extensive, anthropogenic disturbance. The aspen association,

Abstract on the Aspen Association of Northern Michigan

Prepared by: Christopher R . Weber

Joshua G . CohenMichael A . Kost

Michigan Natural Features InventoryP.O. Box 30444

Lansing, MI 48909-7944

For:Michigan Department of Natural Resources

Forest, Minerals, and Fire Management DivisionWildlife Division

September 30, 2006

Report Number 2006-16

Cover Photograph: Young aspen clones in the eastern Upper Peninsula, Michigan. All photos by Christopher R. Weberunless otherwise noted.

Aspen Association Page-1

OverviewThis document provides a brief discussion of the aspenassociation of Michigan, detailing this system’slandscape and historical context, range, ecologicalprocesses, characteristic vegetation and fauna, andthreatened and endangered species. In addition,potential options and strategies are suggested forenhancing biodiversity of managed aspen associationsand for restoring these systems to later successionalforest types.

IntroductionThe aspen association occurs throughout the GreatLakes region as a disturbance dependent vegetationassemblage. Aspen clones and stands of aspen areimportant components of several forested naturalcommunities. The Michigan Natural FeaturesInventory’s (MNFI) classification of naturalcommunities of Michigan (MNFI 2003) does notrecognize the aspen assemblage as a separate naturalcommunity. MNFI does not track early-successionalforested systems. In addition, the focus of MNFI’scommunity classification is on natural communities andthe majority of aspen forests that constitute the aspenassociation have been created and maintained byextensive, anthropogenic disturbance. The aspenassociation, of which the dominant species includetrembling aspen (Populus tremuloides), bigtoothaspen (Populus grandidentata), and paper birch(Betula paperifera), comprises 43% of all pulpwoodharvested in the Great Lakes region (4.2 million cords)(Piva 2006). Aspen forests are also important habitatfor game species such as deer, turkey, woodcock, andgrouse (Gregg and Hale 1977, Kidd and Koelling 1996,McDonald et al. 1998, Nguyen et al. 2004). Thedemand for aspen forest products and early-successional habitat is increasing (Gustafson et al.2003), meanwhile aspen associations haves largelyreplaced later-successional communities in Michigansuch as dry-mesic northern forest and old-growthmesic northern forest (Cohen 2000, 2002, and 2005).The increasing demand for aspen and high proportionof Michigan’s forested landscape occupied by earlysuccessional forest highlights the relevance of areview of the aspen association. This documentprovides a discussion of the history, range,physiographic context, ecological processes, floristicand faunal composition, and biodiversity managementof aspen dominated forests.

RangeTrembling aspen has the most extensive range of anyNorth American tree (Barnes et al. 1998). Its nativerange covers a large swath from the Atlantic Ocean tothe Pacific Ocean, spreading from its southern limit innorthern Illinois, Indiana, Ohio, and Pennsylvania northto the Hudson Bay. Trembling aspen’s range reachesinto the northern and southern Rocky Mountains, andall the way west through Canada and east toNewfoundland and Labrador (Perala 1990).Throughout the south and west Rocky Mountains,trembling aspen is found as a late-successionalspecies, having long-lived clones, sometimes withthousands of stems that expand over large acreages(Barnes et al. 1998). Aspen has been found tocompete with prairies in the Canadian west (Bird1961) and is an integral part of the savanna/forest-prairie transition in Wisconsin and Minnesota (Buelland Buell 1959, Buell and Facey 1960, Cochrane andIltis 2000). In Michigan, trembling aspen approachesthe southern limit of its range, where in undisturbed,old-growth situations it is kept in check by competitionin upland, nutrient rich sites and usually relegated toopen lowlands such as stream or swamp margins(Barnes and Wagner 2004). Bigtooth aspen’s range ismore limited and does not extend west of Minnesotaand Iowa, but reaches northeast through Nova Scotiaand south to Virginia. Northern Lower Michigan iswhere bigtooth aspen has reached its highestabundance throughout its range (Barnes and Wagner2004). Both species of aspen are prevalent in areas ofrecent natural disturbances such as fire or windthrow.

Physiographic ContextAspen’s early-successional life strategy, whichincludes prompt responses to natural andanthropogenic disturbance, results in a widephysiographic range. Researchers have examinedaspen communities with many different climatic andedaphic characteristics. In a comprehensive book onaspen ecology, Graham et al. (1963), discuss thegrowth and form of aspen growing on varioussubstrates including: sandy outwash plains, erodedoutwash/sandy moraines, sandy loams, and seepages.On sandy outwash plains, aspen is in its poorestcondition and usually grows in scattered individualclones. There, soil moisture dictates the shift in thebalance between jack pine (Pinus banksiana) andaspen. Aspen of the highest commercial value is foundon the hills and valleys of eroded outwash plains or onsimilar positions on sandy moraines. Aspen is most


productive on well-drained, nutrient rich sandyloams, but in the absence of disturbance, cannotcompete with other hardwoods. Aspen found onseepages is usually attributed to an exceptionally hotfire during a dry year that burns most of the largeportion of organic soil, leaving the bare mineral soilfor aspen regeneration (Graham et al. 1963).

Other studies have examined aspen on excessively-drained outwash sands and well-drained glacial till(Barnes 1966, Wells 1978, Palik and Pregitzer 1992,Palik and Pregitzer 1995, Peterson and Squiers 1995,Roberts and Gilliam 1995, White et al. 2004), onpoorly-drained morainal sand with a seasonally-highwater table (Roberts and Richardson 1985, Sakai etal. 1985), and on moderately-drained, calcareous tilland glacial lake plain (Lee et al. 1997, Edgar andBurk 2001, Hassett and Zak 2005). Aspen alsooccurs on thin soils overlying bedrock in northeastLower Michigan and throughout the eastern UpperPeninsula. Aspen can be found on a full range oftopographic positions, growing on ridgetops, upper,mid, and lower slopes. Bigtooth aspen is moreprevalent on high slope positions, bedrock ridges, andshallow soil, whereas trembling aspen is moreprevalent on mid and lower slopes with deeper soils.

Vegetation DescriptionAspen is part of several forested systems inMichigan. Because of its diverse array ofphysiographic contexts, vegetation within the aspenassociation varies depending on geography,topography, soil texture, soil drainage, anddisturbance regime. Aspen forest types arenumerous according to NatureServe (2006) andinclude both upland and wetland types. Uplandassociations generally occur on deep, well- torapidly-drained loam soils on a variety of topographicpositions with gentle to moderate slope. The canopyis dominated by trembling aspen, white birch, andbigtooth aspen. Canopy associates include sugarmaple (Acer saccharum), red maple (Acer rubrum),beech (Fagus grandifolia), basswood (Tiliaamericana), red oak (Quercus rubra), black cherry(Prunus serotina), and white ash (Fraxinusamericana). The subcanopy often includesserviceberries (Amelanchier spp.), striped maple(Acer pensylvanicum), and ironwood (Ostryavirginiana). Conifers, usually present in thesubcanopy and shrub layer, include balsam fir (Abiesbalsamifera) and white spruce (Picea glauca).Other common constituents of an aspen understoryand ground layer are eastern white pine (Pinusstrobus) and red pine (Pinus resinosa).

Common shrubs include beaked hazelnut (Coryluscornuta), bush honeysuckle (Diervilla lonicera),American fly honeysuckle (Lonicera canadensis), wildrose (Rosa acicularis), blackberries and raspberries(Rubus spp.), Canada blueberry (Vacciniummyrtilloides), blueberry (Vaccinium angustifolium),sweet fern (Comptonia peregrina), witch-hazel(Hamamelis virginiana), and maple-leaved arrow-wood(Viburnum acerifolium). The herbaceous layer caninclude: wild sarsaparilla (Aralia nudicaulis), big-leavedaster (Aster macrophyllus), maidenhair fern (Adiantumpedatum), bluebead-lilly (Clintonia borealis),bunchberry (Cornus canadensis), wintergreen(Gaultheria procumbens), shining club moss(Lycopodium lucidulum), twin flower (Linnaeaborealis), running ground-pine (Lycopodium clavatum),ground pine (Lycopodium obscurum), Canadamayflower (Maianthemum canadensis), Indiancucumber root (Medeola virginiana), bracken fern(Pteridium aquilinum), starflower (Trientalis borealis),partridge berry (Mitchella repens), naked miterwort(Mitella nuda), pinesap (Monotropa hypopithys),fragrant bedstraw (Galium triflorum), rough-leavedrice-grass (Oryzopsis asperifolia), large-leaved shinleaf(Pyrola elliptica), false spikenard (Smilacinaracemosa), rose twisted stalk (Streptopus roseus),common trillium (Trillium grandiflorum), wildstrawberry (Fragaria virginiana), and violets (Violaspp).

Lowland aspen communities are found on lower slopes,draws, and occasionally on seepages. Soils are deep,poorly-drained, fine-textured, and usually lacustrine inorigin. Dominant trees include trembling aspen andbalsam poplar (Populus balsamifera). Associates caninclude balsam fir, paper birch, white spruce, and burroak (Quercus macrocarpa). The shrub and saplinglayers are usually quite developed and include: balsamfir, tag alder (Alnus rugosa), serviceberries(Amelanchier spp.), red-osier dogwood (Cornusstolonifera), bunchberry, wild currants (Ribes spp.), wildrose, wild red raspberry (Rubus strigosus), and dwarfraspberry (Rubus pubescens). The herb layer includes:wild sarsaparilla, northern heart-leaved aster (Asterciliolatus), big-leaved aster, wood anemone (Anemonequinquefolia), blue-joint grass (Calamagrostiscanadensis), sedges (Carex spp.), blue-bead lily,spinulose woodfern (Dryopteris spinulosa), horsetails(Equisetum spp.), fragrant bedstraw, Canada mayflower,northern bluebell (Mertensia paniculata), and nakedmiterwort. The following rare plants can be found withinthe aspen association: fairy bells (Disporum hookeri,state endangered), heart-leaved arnica (Arnicacordifolia, state endangered), sweet cicely (Osmorhiza


depauperata, state threatened), and rayless mountainragwort (Senecio indecorous, state threatened). (Theabove species lists were compiled from Gates 1930,Kittredge 1938, Westell 1960, Wells 1978, Robertsonand Richardson 1985, Sakai and Sulak 1985, Comer etal. 1995, Palik and Pregitzer 1995, Peterson andSquiers 1995, NatureServe 2006, and MichiganNatural Features Inventory Biotics database).

Natural ProcessesAspen’s natural function in forest ecosystem dynamicsis that of a pioneer species and as such plays a keyrole in natural forest succession. Flourishing after adisturbance, shade-intolerant aspen proliferatesthrough rapid establishment and growth and by out-competing slower growing, shade-tolerant tree speciesin the short term. A hormonal imbalance within theroots of aspen after a disturbance triggers thesprouting of suckers (Schier 1973). The growth ofaspen suckers is enhanced by elevated post-disturbance light levels and resulting increased soiltemperature (Maini and Horton 1966, Prevost andPothier 2002). Aspen suckers usually have very largeleaves and can grow up to eight feet tall in the firstyear (Graham et al. 1963). As an aspen stand matures,a self-thinning process occurs where the multipleaspen clones begin to senesce, leaving fewer, largermature trees (Pollard 1971).

Aspen stands can deteriorate rapidly. Not only areaspen trees short-lived, but some clones are moresusceptible to diseases and fungi such as hypoxyloncanker (Anderson et al. 1997). Healthy stands ofmature aspen can be reduced to a few dying stems injust three to six years (Shields and Bockheim 1981).The remaining trees are subject to increased windexposure and evaporative stress, which tends toaccelerate decline (Peterson and Peterson 1992). Theresult of this rapid decline for the aspen stand is anessential ecological process, that of coarse woodydebris production. Standing dead trees (snags) areutilized as foraging sites, nesting sites, and roosts for anumber of birds and mammals. Dead and downedcoarse woody debris fosters a variety of fungi, mosses,lichens, herbaceous and woody vascular plants as itdecays on the forest floor (Harmon 1986). Aspen logsalso provide a substrate for conifer and hardwoodseedling establishment. Under a natural fire regime inaspen-dominated boreal forests of Alberta, coarsewoody debris can persist for as long as 50-65 yearsafter stand initiation (Lee et al. 1997) with most snagsfalling 10-20 years after disturbance (Lee 1998).Persistence of aspen coarse woody debris is thought tobe shorter in Michigan (i.e., 10-45 years, depending on

site conditions) (Burton Barnes, Dan Kashian, andRonald Murray, personal communication). Coarsewoody debris left by deteriorating aspen stands is alsoa critical component of soil nutrient cycling (Harmon1986, Hafner and Groffman 2005). Along with thecoarse woody debris, shed leaves of the dominantaspen trees are very rich in calcium, and decay quicklyadding essential nutrients to the forest floor. Thenutrients from leaves and coarse woody debris arethen utilized by later-successional species.

Prior to senescence (Photograph 1), the structure ofthe aspen canopy plays a role in successional capacityand understory composition. Mature aspen canopiesare usually open, with crowns that block much lesslight than in mature northern hardwood forests(Photograph 2). This excess light allows an extensiveshrub and forb layer to develop. The early-successional aspen community is quite diverse, asmany shade-intolerant species take advantage of theavailable light. The light allowed through the canopyalso supports advance regeneration of more tolerantspecies, such as hardwoods or conifers, whether bylow stand density on xeric sites (Roberts andRichardson 1985) or by thin aspen crowns on moreproductive sites (Goff and West 1975). As the aspenstand ages, the understory vegetation may compete forresources and reduce soil temperature throughincreased shading, which contributes to the decline ofaspen and further development of canopy gaps (Freyet al. 2004). The later-successional species are thenpoised to replace the mature aspen as disturbancescreate gaps in the aspen canopy.

Photograph 1. Mature aspen on the brink of senescence.(Photo by Michael R. Penskar)


Canopy deterioration and gap creation is a vitaloccurrence in aspen stand development (Hill 2005).The nature of the disturbance, whether it is fire,windthrow, insect outbreaks, or wood decaypathogens, in turn determines the characteristics of thecanopy gap (Frey et al. 2004). Gap size, which isdependent on the severity of the disturbance, willdictate the successional pathway of the individualaspen stand. Large canopy gaps tend to fosterregeneration of intolerant species, essentially aspenreplacing itself (Prévost and Pothier 2003). Smallergaps lead to less understory microclimatic fluctuationand allow more shade-tolerant species to exist (Hill2005). In forests governed by natural disturbanceregimes where succession is allowed to run its course,the patches of aspen exist as a shifting mosaic withinlater-successional communities. Here, naturaldisturbance creates gaps and fosters the developmentof uneven-aged forests (Zhang et al. 1999).

Natural development of aspen stands is disturbancebased. In Michigan, historic natural disturbances werequite common; however the scale and severity of thedisturbance was fundamental to the prevalence ofaspen on the landscape. In northern hardwood forestsof Michigan, for example, the primary manner ofdisturbance was that of minor episodes of wind-throwwhich promoted gap phase dynamics that maintainedshade-tolerant species composition. About 60% of thetrees in the northern hardwood stands entered the

canopy as a result of these minor episodes of lightdisturbance and gap creation (Frelich and Lorimer1991). Episodes of 20% to 50% canopy removal couldbe expected once or twice during the life span of acohort of trees (Frelich and Lorimer 1991). Largestands of aspen were not usually promoted by small,gap-phase dynamics of mature hardwood forests.

Pockets of early-successional habitat, such as aspenforests, typically required major, catastrophicdisturbances such as fire, massive wind-throw, insectepidemics, or flooding (Photograph 3). Thesedisturbances varied by region and local conditions andwhere aspen occurred initially in sparse numbers,these larger scale disturbances resulted in rapidcolonization by aspen because of its ability forsuckering and rapid growth. In northern hardwoodforests, 3% to 15% of the landscape over a 15 yearperiod would have been affected by disturbances of atleast moderate intensity (Lorimer and White 2003).Moderate- and large scale disturbance events resultedin a heterogeneous mixture of species and age classesin which aspen clones colonized many of the mostrecently disturbed locations within a shifting mosaic offorest cover.

Dominance on the LandscapeMany studies have examined the composition of circa1800 forests of Michigan (Whitney 1986, Whitney1987, Palik and Pregitzer 1992, Comer et al. 1995, VanDeleen et al. 1996, Zhang et al. 2000, Leahy andPregitzer 2003). The prevailing view is that aspenplayed a reduced role in circa 1800 forests, comparedto current forests of Michigan. Aspen was not absentfrom the circa 1800 landscape, but rather was a minorcomponent, relegated to isolated areas of naturaldisturbance and stream and swamp margins (Graham

Photograph 2. The relatively open canopy of a sixty-year-oldaspen stand allows for the development of dense and diverseshrub and forb layer. Photograph 3. Large-scale disturbance gaps generated by

windthrow promote regenerating aspen suckers.


et al. 1963, Whitney 1987, Frelich 2002). Near theUniversity of Michigan Biological Station in thenorthern Lower Peninsula, 87% of bearing and linetrees recorded by the original surveyors were hemlock,beech, or white pine (Palik and Pregitzer 1992). Incontrast, present day relative basal areas for bigtoothaspen in the same area was 74% (Palik and Pregitzer1992). Another study in northeastern Lower Michiganshows a shift in acreage of the aspen association landcover type from 0% circa 1800 to 17.2% present day(Leahy and Pregitzer 2003). Comer et al. (1995)estimated circa 1800 aspen cover for the whole stateto occupy 326,769 acres, 0.87 % of the state and0.99% of forested landcover. Aspen cover for theentire state was calculated from IFMAP (IntegratedForest Monitoring, Assessment, and Prescription)landcover data from the year 2000 to be 2,548,842acres (7%), which is approximately 14% of totalforested landcover for the state (Michigan DNR 2000,2001a, 2001b). According to analysis of FIA data inthe Great Lakes, aspen-birch forest has declined sinceits recorded peak in the 1930s by 24%, with theacreage decreasing in Michigan by 37% from 1935 to1993. Despite the trend of decline from the 1930s, thecurrent extent of the aspen association is likely anorder of magnitude greater today than in the circa1800 forests of Michigan (Cleland et al. 2001).

Aspen’s current prevalence within Michigan’slandscape mosaic of early-successional forest is tied tocataclysmic and unprecedented anthropogenicdisturbances of the past (Host et al. 1987, Palik andPregitzer 1992, Zhang et al. 2000, Lorimer 2001, Whiteet al. 2004) and more recent management activities(Karamanski 1989). In the late nineteenth and earlytwentieth century, nearly all of Michigan’s primarywhite pines, eastern hemlocks, and northernhardwoods were removed by extensive logging(Whitney 1987). The vast slash barrens left fromubiquitous logging provided fuel for catastrophic firesthat dominated much of the post-logging era (Graham1963, Karamanski 1989). Most of the competinghardwoods were eliminated by the slash fires and themineral seed bed was perfectly prepared for pineregeneration (Whitney 1987). However, although theoriginal white pines on the drought prone, coarse-textured soils of northern Lower Michigan werethemselves a product of earlier wildfires, the pine seedsource had been removed through extensive logging(Whitney 1987). After logging removed the coniferseed source and the frequent and devastating fires thatfollowed the logging boom eliminated the advanceregeneration of prevalent hardwood deciduous species(Palik and Pregitzer 1992), the stage was set for aspendominance.

Slash fires facilitated aspen dominance in multipleways. Aspen seedlings are very susceptible to groundcompetition, therefore seedling establishment requiresminimal ground vegetation (Barnes 1966). Fire reducesground layer competition by killing or top-killing woodyseedlings and also provides an environment of largelybare mineral soil for seedling establishment. Inaddition, a single, top-killed aspen stem resulting fromfire can produce many suckers all capable of growinginto an adult tree. The dominance of aspen today isthought to be more the result of suckering andsprouting in response to fire, rather than from seedlingestablishment (Barnes 1966).

Current forest and wildlife management practicesoften maintain and expand early-successional forestsystems such as the aspen association throughrepeated, short-rotation harvest regimes. In manyareas, aspen stands that replaced dry-mesic northernforests and mesic northern forests have beenmaintained and even expanded by silviculture andwildlife management geared toward promoting pulpproduction and providing favorable habitat for gamespecies of early-successional forests (particularlywhite tailed deer, turkey, woodcock, and grouse)(Photograph 4).

WildlifeThe aspen association provides habitat to a wide arrayof wildlife and various successional stages of aspenfoster specialized types of organisms. As noted above,some of the game species that utilize the aspenassociation include deer, turkey, woodcock, andgrouse. In addition, the aspen association providesimportant habitat for numerous non-game animals,including several rare species.

Aspen stands are an important food source for white-tailed deer (Odecoilus virginiana). Deer heavilybrowse aspen sprouts, which are not the mostpreferred food for these ungulates. Deer browse inareas of high deer density may actually impede aspenregeneration after final harvest (Westell 1960, Grahamet al. 1963). Aspen’s relatively open canopy allowslight to penetrate to the shrub and herbaceous layers,thus stimulating lush vegetation for deer browse. Themature forests of northern Michigan and throughoutthe Great Lakes, prior to turn of the century logging,were sparsely populated by deer (Alverson et al. 1988,Langenau 1994). The current, widespread, early-successional habitat that has been maintained throughforest and wildlife management practices has helpedmaintain high deer densities in many locations. Thismanagement has had many repercussions throughout


other forest communities in Michigan by increasingdeer browse pressure, specifically impacting diversityof northern hardwoods and cedar swamps, and limitingconifer and oak regeneration (Alverson et al. 1988,Van Deelen et al 1996, Augustine and Frelich 1998,Rooney and Waller 2003, Kraft et al. 2004).

Aspen staminate flower buds are a primary source offood for ruffed grouse (Bonasa umbellus) in winter(Jakubas et al. 1989, Jakubas and Gullion 1991).Grouse density has been highly correlated to use oftrembling aspen as a dominant food supply (Jakubasand Gullion 1991). Ruffed grouse tend to feed on oldertrees that are in poor health (Svoboda and Gullion1972), partly due to the decreased chemical defenseability in older aspen clones (Jakubas and Gullion1991). Small staged clear-cuts in aspen tend toincrease grouse populations by adding a diversity ofage classes and habitat conditions within aspen stands(McDonald et al. 1998). High stem densities of youngaspen stands protect grouse and American woodcock(Scolopax minor) from predators (Dessecker andMcAuley 2001). Woodcock often nest in young aspenstands and utilize moist early-successional forest forforaging (Gregg and Hale 1977, Dessecker andMcAuley 2001). Aspen is also considered high-qualityhabitat for wild turkey (Meleagris gallopavosilvestris) (Nguyen et al. 2004).

Aspen is habitat for numerous non-game mammals.Small mammals such as the red-backed vole(Clethrionomys gapperi) have been found to preferrecently clear-cut aspen stands, due to the amount ofslash, which maintains the vole’s moisture andtemperature requirements during foraging. Alternately,logging slash was found to physically impede foragingin leaf litter for some small mammals, such as themasked shrew (Sorex cinereus) (Yahner 2006). Aspensnags that have decayed heartwood, yet maintainrelatively sound sapwood structure have been found toserve as roosts for big brown bats (Eptesicus fuscus),which will return to the same snag in subsequent years(Willis et al. 2003). Other bats such as the little brownbat (Myotis lucifugus) or the silver-haired bat(Lasionycteris noctivagans) were also found to utilizeaspen stands that were past the mature stage,preferring tall, near-dead or newly-dead trees with lowleaf cover (Crampton and Barclay 1998).

Large, mature aspen trees are selected as nest treesby some raptors. Red-shouldered hawks (Buteolineatus, state threatened) will choose dominant,mature aspen trees with suitable structure to build sticknests (Cooper 1999). Northern Goshawks (Accipitergentiles, state special concern) have also beenobserved utilizing aspen habitat. Forest song birdspecies diversity is enhanced within aspen standsthrough the practice of leaving small (0.5 acre to 2.0acre) uncut patches within aspen clear-cuts thatranged in size from 5 acres to 40 acres respectively(Merrill et al. 1998). Aspen associated with riparianareas provide preferred foraging habitat for beaver(Castor canadensis). Aspen is a favored food ofbeaver and a high proportion of aspen cover in thelandscape can contribute to high beaver populations(Wisconsin Department of Natural Resources 2001)

Biodiversity Management ConsiderationsThis section details opportunities for enhancingbiodiversity of managed and unmanaged aspenassociations at multiple scales and discusses strategiesfor restoring these systems to later successional foresttypes.

From a small-scale biodiversity perspective, aspenstands provide species-rich groundcover and habitatthat is important for numerous wildlife species.However, the issue becomes more complex when oneconsiders ecological diversity and landscape integrity.Logging-induced shifts toward greater dominance ofaspen may enhance stand-scale diversity whilesimultaneously reducing landscape-scale diversity(Reich et al. 2001). Increasing landscape-level

Photograph 4. A twenty-year-old stand of aspen, consideredhigh-quality deer, turkey, and grouse habitat.


biodiversity and ecological function would dictateshifting emphasis towards regenerating more shade-tolerant species and away from maintaining largeaspen clones (Palik et al. 2003). Presently, theecological processes of natural succession are oftenmissing from managed landscapes (Niemelä 1999).Aspen stands that are succeeding to later-successionalforest types such as pine or northern hardwoods areoften harvested to maintain the aspen cover type. Inthis way, silvicultural practices tend to impose astagnant pattern on landscapes (Cohen 2005). Forestrotations under 110 years have no natural precedentand can create an artificial, homogenized landscapewith single cohort stands occupying over 15-25% ofthe landscape (Seymour et al. 2002). In addition, theplacement of aspen stands on the landscape cancontribute to forest fragmentation and excessively highdeer browse pressure in adjacent sites managed forbiodiversity. Where reducing forest fragmentation andincreasing landscape-level biodiversity and ecologicalintegrity are goals, aspen stands can be allowed tosucceed to dry-mesic or mesic northern forests.Similarly, where protection and management of high-quality natural communities or ecological referenceareas is a primary management objective,overbrowsing by deer may be reduced by allowingnearby aspen stands to succeed to late-succesionalforests. Mature upland forests provide deer lesssuitable spring and summer habitat than earlysuccessional forest. Over time, natural plantsuccession of early-successional habitat to moremature forest can lead to the development of lowerquality and quantity of spring and summer deer foodand as trees reach maturity, deer utilize these uplandhabitats less frequently (Verme 1969, Kohn and Mooty1971, Felix et al. 2004).

Current forestry research is emphasizing theimportance of mimicking natural disturbances thatpromote more dynamic patterns of vegetation typesand differing age classes (Zhang et al. 1999, Seymouret al. 2002) and many ecologists believe that theconservation of biological diversity depends on thisapproach (Lorimer 2001). Historically, the naturaldisturbance regime of upland forests created a shiftingmosaic pattern of vegetation across the landscape(Zhang et al. 1999). Frequent small disturbancesresulted in a diverse stand age structure within largeblocks of uneven-aged and even-aged forest.Infrequent catastrophic windthrow and fire createdpatches of early-successional forest that moved acrossthe historical landscape over time. Foresters are nowrealizing the importance of allowing naturalsenescence and succession to take place.

Sustainability of forest resources may depend onallowing natural succession to play a role inmaintaining ecological processes that will ensure longterm forest productivity. Aspen is known to be a verycalcium-demanding species (Paré et al. 1993) thatstores high concentrations of nutrients in perennialtissues (Pastor and Bockheim 1984, Yu and Sucoff2001). Following clear-cutting in aspen stands inMichigan, Richardson and Lund (1975) documentednutrient losses, especially of Calcium and Magnesium,on nutrient-rich soils (i.e., loamy sands). Scientistsacross the Great Lakes region (e.g., from Wisconsin[Boyle et al. 1973] and Minnesota [Silworth and Grigal1982]) and in the southern boreal region of Canada(Bergeron and Harvey 1997) speculate that withconstant regeneration of single cohort aspen stands inthe same location, soil calcium could be depleted,altering long-term nutrient cycling. In addition, short-rotation aspen management in the northern LowerPeninsula and in the western Upper Peninsula ofMichigan has been found to reduce microbial biomassand activity, thus possibly further limiting the futureproductivity of these ecosystems (Hassett and Zak2005). In the western Upper Peninsula, the northernLower Peninsula, and northern Minnesota, repeatedtotal tree harvesting on sandy soils was found toreduce aspen growth and productivity (Stone 2001,Stone 2002). In addition, compaction of the soilresulting from harvest roads and skid trails in northernMinnesota impacts understory vegetation (Berger etal. 2004). Studies in the western Upper Peninsula, thenorthern Lower Peninsula, and northern Minnesotarevealed that soil compaction on fine-textured soils canreduce aspen sucker density, diameter and heightgrowth, and biomass production (Stone and Elioff2000, Stone 2001, Stone 2002).

Study of the boreal aspen forests of Canada haveilluminated some pertinent conclusions as to themanagement of single cohort aspen stands.Researchers suggest the importance of maintainingforested patches within managed landscapes andmanaging towards more diverse mixed wood standsthat reflect natural disturbance regimes. Some arguefor silvicultural practices to be perceived asdisturbance and mimic more closely those of naturalorigin. By favoring species replacement and allowing astand to transition from one type to another, forestswill be more diverse, and therefore more resistant todiseases and exotic insect outbreaks (Bergeron andHarvey 1997). A shift to partial cutting could bettermimic the natural senescence of aspen and generationof canopy openings. Residual networks of remnantforest patches left during partial cutting can help


ensure the survival of interior understory species andspecies sensitive to complete canopy removal (Bergeret al. 2004, Moses and Boutin 2001). Clear-cutssuccessfully repeat those stand-replacinganthropogenic disturbances of the early 1900s that ledto the initial shift in importance towards early-successional forests. However, landscape-level, stand-replacing disturbances are considered to be much rarerthan small within-stand patches of disturbance(Seymour et al. 1999). Canopy gaps in trembling aspenboreal forests are frequent occurrences and present asearly as 60 years following stand origin (Hill et al.2005). Gaps in shorter living aspen stands in Michigancan form within 40 years (Frey et al. 2003). Partialcutting could slowly give way to smaller clear-cuts, asthe portions of intolerant aspen regeneration increasewithin these gaps (Bergeron and Harvey 1997). Thesepractices resemble natural disturbances such asbudworm outbreaks and minor windthrow occurrences(Bergeron and Harvey 1997). Partial cuts in aspenstands do carry with them a trade-off in terms ofproduction potential. Studies in riparian aspen stands innorthern Minnesota indicate that leaving some canopyin order to maintain riparian ecological function,decreases the density and biomass of the aspenproduction (Palik et al. 2003).

Research in boreal aspen forests has also comparedregeneration of aspen after wildfire, a naturaldisturbance, with the regeneration after a clear- orpartial-cut. Although the soil properties were affecteddifferently with the two methods, productivity of aspendid not differ (Pare et al. 2001). The major differencebetween fire and partial or clear-cutting wasstructural, in that fire left many more dead standingtrees (Pedlar et al. 2002, Haeussler and Bergeron2004). Standing dead trees or snags have a great dealof ecological value and are important components ofhealthy forest ecosystems (Harmon et al. 1986). Acrucial step in mimicking natural disturbances in aspenclear-cuts is to leave some mature live and dead stems(Lee et al. 1997, Haeussler and Bergeron 2004).

In many areas, aspen forests that replaced dry-mesicnorthern forests and mesic northern forests have beenmaintained and often expanded by short-rotationsilviculture and wildlife management (Cohen 2005).Self-replacement of aspen in large gaps, either ofnatural or clear-cut origin, has delayed somesuccession to conifers (Cumming et al. 2000). Someresearchers feel restoring white pine where it has beenreplaced in the landscape by aspen will be difficult dueto competing aspen roots and white pine blister rust(Whitney 1987, Peterson and Squiers 1995). However,

other research suggests that early-successional forest,especially aspen stands, can function as nurse cropsfor the rehabilitation of late-successional forests(Curtis 1959, Mosseler et al. 2003) (Photographs 5 and6). A positive relationship has been observed betweenwhite pine growth rate and local aspen abundance asaspen stands age (Peterson and Squiers 1995). Inaddition, white pine is the most dominant species ofadvance regeneration under some big-toothed aspenstands and does not necessarily require a canopy gapfor successful establishment and growth (Palik andPregitzer 1995).

Allowing early-successional stands of aspen and paperbirch to revert back to late-successional forest wouldrequire the employment of the management tool ofpatience (Cohen 2005). By allowing aspen stands, oldfields, and wildlife openings to succeed to white pine,managers can re-establish the prevalence of the whitepine seed source throughout the landscape and developsites for eventual mature forest re-establishment.Forests of the Great Lakes are becoming increasinglyfragmented (Mladenoff et al. 1993, Heilman et al.2002, Bresse et al. 2004), and forestry and wildlifemanagement practices that focus on species- andstand-based management have directly and indirectlypromoted landscape fragmentation and exacerbatededge effects (Bresse et al. 2004). Allowing early-successional aspen forest to succeed to late-successional dry-mesic northern forest and mesicnorthern forest will not only increase uncommon foresttypes in the Great Lakes landscape, it can also helpdampen the effects of forest fragmentation.

Photographs 5 and 6. Aspen stands can function as nursecrops for conifer regeneration. (Photograph 6 by Joshua G .Cohen)


Research NeedsDemand for aspen products (e.g., paper, lumber, woodcomposite building material, and biofuel) and aspen aswildlife habitat is increasing at the same time there areincreasing pressures to enhance the non-commodity,non-game benefits of aspen forests. Aspenmanagement involves a delicate balance betweeneconomic, social, and ecological values. One methodof mitigating the strain on this resource is to researchwhere aspen is grown in relation to where it wouldhave the greatest growth potential (Gustafson et al.2003). Perhaps the total acreage of aspen coulddecline if stands managed for aspen were moreproductive. However, the most productive aspenstands may also be high-quality sites for diverse late-successional forest. This idea raises importantquestions: How can resource practitioners make aspenproduction and management more compatible withecological goals? Where are aspen stands in relation to1) areas of high site index, 2) areas of limited forestfragmentation, and 3) areas of high biodiversity?Aspen management is thought to increase deerpopulations which, in turn, have many deleteriouseffects on adjacent native communities because ofincreased deer browse pressure (Westell 1960,Alverson et al. 1988, Van Deelen et al 1996, Augustineand Frelich 1998, Wisconsin Department of NaturalResources 2001, Horsley et al. 2003, Rooney andWaller 2003, Kraft et al. 2004). Will high-productionaspen sites be located near ecological reference sites,specifically large tracts of congruent high-qualitynatural areas? Can fragmentation of natural forestedareas be limited by allowing lower productivity aspenstands to succeed into mature forests? An investigationinto location, productivity, and ecological setting ofareas of aspen management could lead to moreeffective balancing of the economical, social, andecological uses of this resource.

Additional research could examine the intensity withwhich aspen is harvested. How does repeated, short-rotation management, which has no natural disturbanceprecedent in terms of intensity and return interval,affect nutrient dynamics, floristic and faunalcomposition, site potential, and late-successional seedsources at multiple scales? What are the landscape-level effects of current management practices thatutilize stand-replacing procedures? Research aimed atthese and similar questions will contribute tosustainable forest management practices and the long-term viability of Michigan forests.

Other Classifications:

Michigan Natural Features Inventory (MNFI):Mesic northern forest, dry-mesic northern forest, drynorthern forest

Michigan Natural Features Inventory Circa 1800Vegetation: Aspen-Birch Forest (413).

Michigan Department of Natural Resources(MDNR): A-Aspen

Michigan Resource Information Systems(MIRIS): 410 (Broadleaved Forest/Upland DeciduousForest), 4131 (Aspen/Birch), 4132 (Bigtooth Aspen),4133 (Trembling Aspen), 4134 (UndifferentiatedAspen/White Birch), 414 (Wetland Deciduous/Lowland Forest), and 4142 (Aspen)

Integrated Forest Monitoring, Assessment, andPrescription (IFMAP): Aspen Association

NatureServe/Nature Conservancy NationalClassification:CODE; ALLIANCE; ASSOCIATION; COMMONNAME

I.B.2.N.b; Populus tremuloides - Betula papyriferaForest Alliance; Populus tremuloides - Betulapapyrifera / (Abies balsamea, Picea glauca) Forest;Aspen - Birch / Boreal Conifer Forest

I.B.2.N.b; Populus tremuloides - Betulapapyrifera Forest Alliance; Populus tremuloides -Betula papyrifera - (Acer rubrum, Populusgrandidentata) Forest; Aspen - Birch - Red MapleForest

I.B.2.N.b; Populus tremuloides - Betula papyriferaForest Alliance; Populus tremuloides - Betulapapyrifera / Acer saccharum - Mixed HardwoodsForest; Aspen - Birch / Sugar Maple - MixedHardwoods Forest

I.B.2.N.d; Populus tremuloides Temporarily FloodedForest Alliance; Populus tremuloides - Populusbalsamifera - Mixed Hardwoods Lowland Forest;Aspen - Balsam Poplar Lowland Forest

I.C.3.N.a Picea glauca - Abies balsamea –Populus spp. Forest Alliance; Picea glauca - Abiesbalsamea - Populus tremuloides / Mixed HerbsForest; Spruce - Fir - Aspen Forest


I.C.3.N.a Pinus strobus - (Pinus resinosa) -Populus tremuloides Forest Alliance; Pinus strobus -Populus tremuloides / Corylus cornuta Forest;White Pine - Aspen - Birch Forest

II.B.2.N.a; Populus tremuloides Woodland Alliance;Populus tremuloides - (Populus grandidentata)Rocky Woodland; Mixed Aspen Rocky Woodland

Related Abstracts: Red-shouldered hawk, northerngoshawk, mesic northern forest, dry-mesic northernforest, dry northern forest

AcknowledgementsFunding for this abstract was provided by the MichiganDepartment of Natural Resources’ Forest, Mineral,and Fire Management Division and Wildlife Division.Numerous MNFI staff assisted with the creation ofthis paper. Kraig Korroch assisted with final reportformatting. Dave Cuthrell, Brittany Woiderski, andMichael “Cletus” Beulow provided insight and fieldassistance. Jeff Lee provided the use of “Ol’Bessy.”Helen Enander provided GIS support. Lyn Scrimgerserved as the grant administrator and Sue Ridge,Connie Brinson, and Patrick Brown providedadministrative support. Laura Heinemann, MarthaGove, Patrick Brown, Cara Boucher, Patrick Lederle,Michael Donovan, Lawrence Pedersen, JasonStephens, Ronald Murray, and David Neumannprovided essential editorial input.

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Abstract on the Aspen Association of Northern Michigan · 2018. 10. 2. · association have been created and maintained by extensive, anthropogenic disturbance. The aspen association,